A known electromagnetic clutch is configured to electrically control a power transmission and interruption relative to a water pump or the like that circulates a cooling water at an engine for a vehicle. Such electromagnetic clutch is disclosed in JPH03-28527A (which will be hereinafter referred to as Reference 1). The electromagnetic clutch disclosed in Reference 1 includes a rotor having a drum shape and being rotatably supported by a compressor housing at which a compressor rotation shaft serving as a rotational shaft of the compressor housing is provided. A frictional surface is integrally formed at the rotor while being positioned perpendicular to a rotational axis of the rotor (the electromagnetic clutch). An exciting coil is arranged at an inner side of the rotor, specifically, at an inner side relative to the frictional surface. In addition, an armature is connected via a plate spring to an inner boss that is connected to the compressor rotation shaft. The armature is arranged to face the frictional surface of the rotor.
According to the electromagnetic clutch disclosed in Reference 1, a power supply to the exciting coil causes the armature to be magnetically attracted in a direction towards the rotor. Then, the plate spring is elastically deformed so that the armature is moved to the rotor to thereby tightly make contact with or be attracted to the frictional surface of the rotor. As a result, the rotor and the armature integrally rotate, thereby transmitting a rotational force of the rotor to the compressor rotation shaft.
JP2001-200860A (which will be hereinafter referred to as Reference 2) discloses an electromagnetic clutch (friction clutch) including a rotor that is rotatable about a rotational axis of the electromagnetic clutch and a driven shaft that is also rotatable about the rotational axis of the electromagnetic clutch. The rotor integrally rotates with a pulley. An intermediate member is supported by a boss of the driven shaft so as to be rotatable about a pin that is positioned in parallel to the rotational axis. The intermediate member rotates about the pin so as to be switchable between a state where the intermediate member is in contact with an outer peripheral surface of the rotor and a state where the intermediate member is away from the outer peripheral surface of the rotor. An electromagnetic coil is provided within the rotor so as to magnetically attract the intermediate member in a direction towards the rotor (i.e., in an inward direction of the electromagnetic clutch). In addition, a spring is provided at the electromagnetic clutch so as to bias the rotor towards an inner peripheral surface of the pulley.
According to the electromagnetic clutch disclosed in Reference 2, an electric power is supplied to the electromagnetic coil to thereby attract the intermediate member towards the rotor against a biasing force of the spring. The intermediate member makes contact with the outer peripheral surface of the rotor accordingly. Then, the intermediate member is further pressed against the outer peripheral surface of the rotor by a torque from the rotor, which leads to a power transmission state in which the rotational force of the rotor is transmitted to the driven shaft. On the other hand, the supply of the electric power to the electromagnetic coil is stopped so that the intermediate member is separated from the outer peripheral surface of the rotor by the biasing force of the spring. As a result, a power interruption state in which the power transmission of the rotational force of the rotor is interrupted is obtained.
The electromagnetic clutch disclosed in Reference 1 includes a configuration in which a driving member makes contact with a driven member by the power supply to the exciting coil to thereby achieve the power transmission. Therefore, in order to maintain the secure power transmission, an increased power supply to the exciting coil may be inevitable.
The electromagnetic clutch disclosed in Reference 2 includes a configuration in which the intermediate member is magnetically attracted to the outer peripheral surface of the rotor by the power supply to the coil. After the intermediate member makes contact with the outer peripheral surface of the rotor, the rotational force of the rotor acts as a force in a direction where the intermediate member makes contact with the outer peripheral surface. As a result, the intermediate member strongly makes contact with the outer peripheral surface of the rotor, thereby achieving the secure power transmission.
However, the electromagnetic clutch is provided not only to achieve the switching between the secure power transmission and the secure power interruption but also to achieve such switching with less power supply. Specifically, in a case where the electromagnetic clutch is provided at a water pump, a radiator fan, or the like for a vehicle engine and therefore the water pump or the radiator fan is brought in a driving state for a longer period of time than in a non-driving state, and a continuous power supply is required in the driving state, an improvement may be necessary in view of energy saving.
A need thus exists for an electromagnetic clutch which is not susceptible to the drawback mentioned above.